1. Field of the Invention
This application relates generally to albedo control and more specifically, to a process of reducing the albedo of a molybdenum article through a process of oxidation.
2. Description of the Prior Art
One of the proposed uses of molybdenum articles is in optical display units which have conventionally used stacked nixie tubes and the like. In order to use molybdenum articles in optical display units, it is necessary to reduce the albedo of the molybdenum article. One way of reducing the albedo of the surface of the molybdenum article is to oxidize the surface. However, the process of oxidation not only reduces the albedo of a molybdenum article but oftentimes changes the physical characteristics of the molybdenum in the article.
A typical prior art oxidation process is generally described in the Kosco U.S. Pat. No. 3,577,275. Kosco states that to oxidize tungsten or molybdenum one can use temperatures as low as 800.degree. F. if very long time periods are permissible and temperatures as high as 1800.degree. F. if short times are permissible. Kosco states the preference being oxidizing in the air for 1 to 2 hours at about 1500.degree. F. An analysis of oxidized articles made under these times and temperatures reveals that physical change occurred to the articles which rendered the article unsuitable for its ultimate use. For example, it was found that Kosco's suggested temperature and times causes the molybdenum article to become brittle. Also at high temperature a white coating was produced on the surface of the molybdenum article. Obviously, the white surface does not have the proper albedo.
The Marden et al U.S. Pat. No. 1,861,637 discloses a process of oxidation in connection with the production of alkali metal tubes. After a cleaning process, an oxide layer is formed on either a tungsten or molybdenum filament by exposing the filaments to an oxygen atmosphere having atmospheric pressure at a temperature of approximately 2000.degree. C. for 1/2 minute. Marden suggests that instead of oxygen, ordinary atmospheric air may be used but this requires a pressure of from 3 to 4 times that of the oxygen.
The VanGessel U.S. Pat. No. 1,699,639 shows a method for forming an oxide layer on a tungsten wire by heating in air at a temperature of about 700.degree. C. and then exposing to a reducing vapor.
The Foulke U.S. Pat. No. 1,628,456 discloses a method of forming an oxide layer on tungsten or other wire by exposing to a temperature of 850.degree. C. to 950.degree. C. for approximately 0.002 minutes.
The present process is the discovery of a range of relationships which produce an oxidized surface which has the necessary albedo reduction without affecting the physical characteristics of the article.